Re: [Discuss-gnuradio] saturation with multi_fft.py

[Eric Blossom]

On Fri, Oct 05, 2007 at 05:19:38PM -0400, address@hidden wrote:
> Can anybody tell me why the multi_* versions of the fft and scope 
> applications (found in the multi-antenna directory) seem to have such a 
> large gain on the input signal for high decimation values (eg 250), 
> relative to the non-multi versions (at same -g gain setting) of the fft 
> and scope aps?  I'm saturating the inputs for anything above .3 V p-p 
> using a function generator, whereas the non-multi versions work fine up to 
> the 2 V p-p limit of the A/D.
> 
> I'm putting in frequencies of 10 kHz and tuning to 0 Hz.  I also noticed 
> that the magnitudes of the values displayed by the multi_* programs is 
> dependent on the decimation.  At lower decimation values the magnitudes 
> are much less.  This variation is not as pronounced on the non-multi 
> versions, although there is a 'dip' in the response.  To give you some 
> idea of the magnitude difference:
> 
> for a .3 V p-p 10 kHz sine-wave input, here are the peak values:
> 
> decimation       usrp_fft.py (with -S)         multi_scope.py
> 64                     1750                          1750
> 128                    1750                          1750
> 144                    1750                          2900
> 188                    1000                          8000
> 200                    1200                          10000
> 250                    1750                          25000
> 
> So for decimations greater than 128 the mult_* applications cannot take 2 
> V p-p. Any ideas?
> 
> thanks,
> eric

The FPGA rbf's checked into the trunk are probably not up to date for
the multi_scope case.  I'm not in the habit of building multi-board
versions since I'm not sure how to test them.

If you've got the free (beer) Quartus tools installed, you can rebuild
the rbf's and check.

Eric